A casting assembly consists of a pouring cup, a gating system (including downsprues, choke, and runner), risers, sleeves, molds, cores, and other components. To produce a metal casting, metal is poured into the pouring cup of the casting assembly and passes through the gating system to the mold and/or core assembly where it cools and solidifies. The metal part is then removed by separating it from the core and/or mold assembly.
Risers or feeders are reservoirs which contain excess molten metal which is needed to compensate for contractions or voids of metal which occur during the casting process. Metal from the riser fills such voids in the casting when metal from the casting contracts. Thus, the metal from the riser is allowed to remain in a liquid state for a longer period, thereby providing metal to the casting as it cools and solidifies. Sleeves are used to surround or encapsulate the riser and other parts of the casting assembly in order to keep the molten metal in the riser hot and maintain it in the liquid state. The temperature of the molten metal and the amount of time that the metal in the riser remains molten are a function of the sleeve composition and the thickness of the sleeve wall, among other factors.
Typical materials used to make sleeves are aluminum, oxidizing agents, fibers, fillers and refractory materials, particularly alumina, aluminosilicate, and aluminosilicate in the form of hollow aluminosilicate spheres. The type and amount of materials in the sleeve mix depends upon the properties of the sleeves that are to be made, particularly the insulating and exothermic properties of the sleeve.
Three basic processes are used for the production of sleeves, "ramming", "vacuuming", and "blowing or shooting". Ramming and blowing are methods of compacting a sleeve composition and binder into a sleeve shape. Ramming consists of packing a sleeve mix (sleeve composition and binder) into a sleeve pattern made of wood, plastic, and/or metal. Vacuuming consists of applying a vacuum to an aqueous slurry of a refractory and/or fibers and suctioning off excess water to form a sleeve. Typically, when vacuuming is used to form the sleeve, the sleeves formed are oven-dried to remove contained water and cure the sleeve. If the contained water is not removed, it may vaporize when it is exposed to the hot metal and result in a safety hazard.
These compositions are modified, in some cases, by the partial or complete replacement of the fibers with hollow aluminosilicate microspheres. See PCT publication WO 94/23865. This procedure makes it possible to vary the insulating and exothermic properties of the sleeves and reduces or eliminates the use of fibers which can create health and safety problems to workers making the sleeves and using the sleeves in the casting process. WO 98/03284 discloses a cold-box and no-bake process for making sleeves with certain hollow aluminosilicate microspheres. The hollow aluminosilicate microspheres disclosed in these patent applications are of two basic types. One type is high in alumina (at least 38 weight percent based upon the weight of the microspheres) and contains small amounts of alkaline impurities. The other type is low in alumina (less than 38 weight percent alumina based upon the weight of the microspheres), high in silica, and contains a higher amount of alkaline impurities, such as Na, K, Ca, Mg. The low alumina hollow microspheres release alkaline materials when they are heated to high temperatures of about 700.degree. C. and higher. These resulting alkaline materials are not desirable by-products. These decomposition products contaminate the sand and the sand is less effectively bonded when used again to make molds and cores for use in the casting assembly.